Nanofiltration is a pressure-driven membrane filtration process, falling between reverse osmosis and ultrafiltration. Separation mechanisms in nanofiltration vary between membranes because of material, tightness and their interactions with solutes and solvents of different kinds. Retention in nanofiltration is explained partly by sieving and partly by non-sieving mechanisms. The sieving (size exclusion) is often explained by differences in molar mass of solute or the molecular dimensions and pore size (or free volume) in the membrane structure. Although the sieving is often the dominating phenomenon to restrict the permeation of compounds it seldom explains the retention of salts or does not always explain the retention of organic compounds. Electrostatic repulsion, polarity, dielectric exclusion, hydrophobicity/hydrophilicity, etc. are known to affect the separation of different compounds.
Nanofiltration typically retains large and organic molecules with a molar mass greater than 300 g/mol. The most important nanofiltration membranes are composite membranes made by interfacial polymerisation. Polyether sulfone membranes, sulfonated polyether sulfone membranes, polyester membranes, polysulfone membranes, aromatic polyamide membranes, polyvinyl alcohol membranes and polypiperazine membranes are examples of widely used nanofiltration membranes. Inorganic and ceramic membranes can also be used for nanofiltration.
Nanofiltration membranes have been defined by their ability to reject only ions which have a negative charge over one, such as sulphate or phosphate, while passing single-charged ions. Another distinctive feature is their ability to reject uncharged, dissolved materials and positively charged ions according to the size and shape of the molecule in question. The nominal cut-off value of the molecular size relating to nanofiltration is defined to be in the range of 100-1000 g/mol.
Negative retention in nanofiltration has been observed earlier when salt solutions are filtered. A negatively charged membrane repels negatively charged ions and the higher is the charge of the ion the better it is repelled. This means that e.g. divalent sulphate ions are better retained than monovalent chloride ions. In order to maintain the electroneutrality on both sides of the membrane the permeation of chloride ions increases and even negative retention of chloride ions can be achieved (Donnan phenomenon). The permeation of chloride ions can be facilitated by adding more sulphate ions. When the proportion of sulphate and chloride ions increases the permeation of chloride ions increases. In other words when the amount of better retaining compounds increases the retention of less retained compounds decreases.
The effects of an organic ion on the nanofiltration separation of inorganic salts have been studied by I. Koyuncu and D. Topacik, in Journal of Membrane Science 195 (2002) 247-263.
The effect of inorganic salts on the nanofiltration separation of organic compounds has been studied by G. Bargeman et al., in Journal of Membrane Science 247 (2005) 11-20. It was found that the presence of salts ions, especially those for which the membrane show low retention, leads to reduction of the retention of neutral components such as glucose. The retention reduction was dependent on the membrane selected.
W. Koschuh et al., in Journal of Membrane Science 261 (2005) 121-128, found considerably lower retention values for arabinose, glucose, fructose and sucrose using PES004H (Nadir) and Inocermic (Inocermic/D) nanofiltration membranes than using PES10 (Nadir) N30F (Nadir) and MPF36 (Koch) nanofiltration membranes in nanofiltrating silage juice but no significant separation of a target compound was achieved.
G. Laufenberg et al., in Journal of Membrane Science 110 (1996) 59-68, has studied the effects of several carboxylic acids on the rejection of acetic, propionic and formic acids in binary and tertiary solutions in a reverse osmosis process.
U.S. Pat. No. 6,177,265, Roquette Freres (published Jan. 23, 2001) relates to a process for the manufacture of a starch hydrolysate with a high dextrose content. In this process, a starch milk is subjected to enzymatic treatment to obtain a raw saccharified hydrolysate. The hydrolysate thus obtained is then subjected to nanofiltering to collect as the nanofiltration permeate the desired starch hydrolysate with a high dextrose content. An improved dextrose enrichment of permeate was observed when the saccharified hydrolysate to be nanofiltered was not demineralised.
U.S. Pat. No. 6,406,546 B1, Tate & Lyle Industries (published Jun. 18, 2002) discloses a process of obtaining sucrose from a sucrose-containing syrup by nanofiltering the syrup through a nanofiltration membrane and recovering the nanofiltration retentate enriched in sucrose. It is recited that invert sugars are passed through the nanofiltration membrane into the nanofiltration permeate.
U.S. Pat. No. 5,965,028, Reilly Industries (published Oct. 12, 1999) discloses a process for the separation of citric acid from less desirable components having a molecular weight similar to that of citric acid (such as glucose and/or fructose) by nanofiltration. A nanofiltration permeate enriched in citric acid is recovered. The feed used for the nanofiltration is typically a clarified citric acid fermentation broth.
WO 02/053783 and WO 02/053781, Danisco Sweeteners Oy (published Jul. 11, 2002) disclose a process of producing a xylose solution from a biomass hydrolysate by subjecting the biomass hydrolysate to nanofiltration and recovering as the nanofiltration permeate a solution enriched in xylose. The feed used for the nanofiltration may be for example a spent sulphite pulping liquor containing a mixture of other closely-related monosaccharides, such as glucose, galactose, rhamnose, arabinose and mannose, in addition to the desired xylose. It was found that the nanofiltration effectively concentrated pentose sugars, such as xylose in the nanofiltration permeate, while hexose sugars remained in the nanofiltration retentate. However, the permeate obtained from the nanofiltration had a relatively low dry substance content (1 to 2%) and consequently a low xylose content. Furthermore, the xylose yields were low (less than 20%). Hereby the performance of the process was not sufficient for industrial operation.
Thus, nanofiltration has been used for separating neutral organic compounds such as monosaccharides like glucose from di- and higher saccharides. In addition, it has been used to separate citric acid from glucose and/or fructose. However, when nanofiltration is used to separate neutral organic compounds such as monosaccharides from each other so that the compound to be recovered is found in the permeated liquid, water has to be added to the retentate side to facilitate sufficient product yield in the permeate. This, however, leads to considerable dilution of the product compound in the permeate side.